1. Field of the Invention
The present invention relates to a method of carrying a boat supporting a number of objects such as semiconductor wafers, into and out of a thermal treatment furnace. It also relates to an apparatus for carrying a boat supporting semiconductor wafers, into and out of a thermal treatment furnace.
2. Description of the Related Art
Semiconductor devices, such as ICs and LSIs, are manufactured by performing various processes on semiconductor wafers sliced from an ingot. Among these processings are: treating the surface of each wafer; diffusing impurities into the wafer; depositing layers on the wafer; and etching the layers. These processings involve heating the wafer. Hence, each wafer is heat-treated several times during the manufacture of the semiconductor devices.
Generally, to heat-treat semiconductor wafers, use is made of a so-called multi-stage furnace, which comprises two or more furnace units mounted one upon another, for example, a four-stage furnace. Each furnace unit contains a process tube made of quartz and positioned horizontally. A soft-landing loader is located in front of each furnace unit, for loading a boat (here-inafter called "wafer boat"), which supports wafers, into the process tube, and unloading the wafer boat from the process tube.
Various types of soft-landing loaders are disclosed in Japanese Patent Laid-Open No. 62-3571, U.S. Pat. No. 4,008,815, and U.S. Pat. No. 4,468,195. Each of these soft-landing loaders comprises a fork. The fork extends toward the entrance of a process tube, in the axial direction of the tube (hereinafter referred to as "X-axis direction). The fork is shaped like a hollow, and has a distal end so shaped as to support a wafer boat stably.
The soft-landing loader further comprises an X-axis drive mechanism for moving the fork in the X-axis direction, a .theta.-oscillation mechanism for oscillating the distal end of the fork up and down, a controller for controlling the movement of the fork, and a computer system for controlling the controller in accordance with a specific program.
The wafer boat is loaded into the process tube in the following way. First, the X-axis drive mechanism is operated, thereby moving the fork toward the tube for a predetermined distance L. Then, the .theta.-oscillation mechanism is operated, thus slowly oscillating downward the distal end of the fork, whereby the wafer boat slides from the fork into the process tube. The wafer boat is unloaded from the process tube in the following manner. First, the X-axis drive mechanism is operated, thereby moving the fork into the process tube. Then, the .theta.-oscillation drive mechanism is operated such that the distal end of the fork is slowly oscillated upward, thus holding and lifting the wafer boat upward. Finally, the X-axis drive mechanism is actuated, and moves the fork away from the tube for the predetermined distance L. During the loading and unloading of the wafer boat, the controller controls the fork such that the fork is kept out of contact with the inner wall of the process tube which is made of quartz. Since the fork does not rub the inner wall of the tube, no quartz powder comes off the inner wall of the tube and attaches to the semiconductor wafers mounted on the wafer boat.
In the conventional method of loading the wafer boat into the process tube, and unloading the boat from the process tube, however, the fork and the wafer boat assume one positional relationship during the loading of the boat, and take another positional relationship during the unloading of the boat. This is inevitably because the .theta.-oscillation mechanism inclines the wafer boat which is inclined to the axis of the process tube when the boat is loaded into the process tube, and also because the X-axis drive mechanism does not always move the boat in the same way in both loading and unloading the wafer boat. Consequently, the unloaded wafer boat takes a position different from the position it assumed before being loaded into the process tube. Unless the boat is placed in the same position as it was before loaded into the process tube, it cannot be easily transported to the position where the wafers mounted on it will undergo the next processing.